teristic of the majority of the above-mentioned fanriilies .whereas among 

 Gastrocotylidae and Microcotvlidae the number of clannps varies from six 

 pairs upwards, whereas the ciiitinous hooks, for the most part, number four 

 pairs or are absent in the adult. 



External Covers. External covers of monogenetic trematodes p. 40 

 are represented by a cuticle typical for the parasitic flatworms (Fig. 55), 

 for the most part double-layered, smooth, and comparatively thin, under 

 which is located a more or less well-developed basal membrane which, in 



a number of cases, is poorly visible. 



Because of this poor visibility, 

 certain authors question its presence 

 altogether. As a rule, subcuticular 

 cells are absent in the nnonogenetic 

 trematodes; if in rare cases they 

 exist then they are in relatively small 

 numbers and are under and partially 

 between the fibers of the longitudinal, 

 circular and diagonal muscle layers. 



As among other Cerco- 

 meromorpha, the question of the 

 covering of the monogenetic trema- 

 todes is very complex and there is 

 no commonly accepted opinion con- 

 cerning its origin. The most wide- 

 spread is that the cuticle represents 

 a derivative of the ectodermal epithel- 

 ium which, as a result of its adapta- 

 tion to parasitism receded into the 

 body and is represented by subcuti- 

 cular cells or so-called submerged 

 epithelium. This point of view is 

 shared by many zoologists. The com- 

 parison of the structure of the cover- 

 ings in Cercomeromorpha and Tur- 

 bellaria serves as a basis for this. 

 Thus, in a series of Acoela and 

 Triclada the receding of epithelial 

 cells into the body under the dermal 

 musculature is observed, that is, 

 one observes the relationships which 

 are close to those which are seen among parasitic flatworms. Analogous 

 views can be seen among parasitic Myzostomidae (Fedotov, 1915). How- 

 ever, other points of view exist. Thus, Monticelli, (Monticelli, 1893) sup- 

 posed that the cuticula represents an ectoderm which underwent conaplete 

 metamorphosis and which has a changed protoplasm, missing nuclei and 

 missing delineations between the cells. This supposition is based on the 



Fig. 54. Dichlidophora denticulata 

 (Olsson), adult worm from the gills 

 of PoUachius virens (L. ) from the 

 Barents Sea. 



34 



